The present invention relates to household refrigerators operating with a vapor compression cycle and more particularly, to refrigerators with a three stage compressor.
Currently produced household refrigerators operate on the simple vapor compression cycle. The cycle includes a compressor A, condenser B, expansion throttle C, evaporator D, and a two phase refrigerant. In the prior art refrigerator cycle of FIG. 1, a capillary tube acts as an expansion throttle. The capillary tube is placed in close proximity with the suction line of the compressor to cool the capillary tube. The subcooling which occurs to the refrigerant in the capillary tube increases the cooling capacity per unit mass flow rate in the system thereby increasing system efficiency which more than compensates for the disadvantage of increasing the temperature of the gas supplied to the compressor. The evaporator in FIG. 1 operates at approximately -10.degree. F. Refrigerator air is blown across the evaporator and the air flow is controlled so that part of the air flow goes to the freezer compartment and the remainder of the flow goes to the fresh food compartment. The refrigerator cycle, therefore, produces its refrigeration effect at a temperature which is appropriate for the freezer, but lower than it needs to be for the fresh food compartment. Since the mechanical energy required to produce cooling at low temperatures is greater than it is at higher temperatures, the simple vapor compression cycle uses more mechanical energy than one which produces cooling at two temperature levels.
A well known procedure to reduce mechanical energy use is to operate two independent refrigeration cycles, one to serve the freezer at low temperatures and one to serve the fresh food compartment at an intermediate temperature. Such a system, however, is very costly.
Another problem which occurs in cooling for freezer operation in the simple vapor compression cycle, is the large temperature difference between the inlet and outlet temperatures of the compressor. The gas exiting the compressor is superheated, which represents a thermodynamic irreversibility which results in a relatively low thermodynamic efficiency. Lowering the amount of superheat will provide for decreased use of mechanical energy and therefore greater efficiency.
One solution to these problems is disclosed in U.S. Pat. No. 4,910,972 which is assigned to the same assignee as the present invention. U.S. Pat. No. 4,910,972 discloses a dual evaporator two stage cycle suitable for use in household refrigerators. The system comprises a first expansion valve, a first evaporator for cooling the freezer compartment, a first compressor, a second compressor, a condenser, a second expansion valve, and a second evaporator for cooling the fresh food compartment. All of the above elements are connected together in series in that order, in a refrigerant flow relationship. A phase separator connects the second evaporator to the first expansion valve and provides intercooling between the first and second compressors.